HR: 18:05h
AN: P44A-08    [Abstracts]
TI: Pickup Ion Phase Space Distributions at Titan: Effects of Atmospheric Spatial Gradients
AU: * Sittler, E C
EM: edward.c.sittler@nasa.gov
AF: NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Code 673, Greenbelt, MD 20771, United States
AU: Hartle, R E
EM: richard.e.hartle@nasa.gov
AF: NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Code 673, Greenbelt, MD 20771, United States
AB: The composition and structure of neutral exospheres imbedded in moving plasmas can be determined by measurements of the velocity distributions of their pickup ion progeny. In turn, the velocity distributions are dependent on the spatial structure of the neutral source gases. Since Titan's neutral exosphere extends into the Saturn's magnetosphere (or solar wind) and well above its ionopause, it serves as a good place to analyze such characteristics. They are analyzed using pickup ion measurements made by the Cassini Plasma Spectrometer (CAPS) at Titan [e.g., Hartle et al., 2006] and an ion kinetic model. The model [Hartle and Sittler, 2007] is an expression describing the phase space density of pickup ions, which is derived from the Vlasov equation with an ion source that explicitly accounts for the velocity and spatial variation of the exosphere source gases. A fundamental parameter in the phase space density expression is the ratio of the gyroradius to the neutral scale height., α = rg/H. Titan's exosphere includes H, H2, CH4 and N2, with scale heights near the exobase of ~ 2400, 1200, 149 and 85 km, and α of ~ 0.1, 0.4, 27, and 82, respectively. This structured exosphere yields pickup ions whose phase space distributions are beam-like when α >> 1 and fluid- like when α << 1. Downstream from the source peak, the light pickup ions, with α << 1, are easily observed because the phase space density is almost uniform over the orbit phases. On the other hand, the phase space distribution of the heavier ions, with α >> 1, peaks over a narrow velocity and spatial range. This beam-like nature makes it more difficult to observe the heavy ions because the downstream positions and viewing directions are constrained. Examples of these extremes will be discussed. Hartle et al., Planet. Space Sci. vol. 54, 1211, 2006 Hartle and Sittler, J. Geophys. Res., in review, 2007
DE: 0328 Exosphere
DE: 2732 Magnetosphere interactions with satellites and rings
DE: 5210 Planetary atmospheres, clouds, and hazes (0343)
DE: 6281 Titan
DE: 7807 Charged particle motion and acceleration
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly